Analytical device

ABSTRACT

A sensor for chemical species or biological species or radiation presenting to test fluid a polymer composition comprises polymer and conductive filler metal, alloy or reduced metal oxide and having a first level of electrical conductance when quiescent and being convertible to a second level of conductance by change of stress applied by stretching or compression or electric field, in which the polymer composition is characterized by at least one of the features in the form of particles at least 90% w/w held on a 100 mesh sieve; and/or comprising a permeable body extending across a channel of fluid flow; and/or affording in-and-out diffusion of test fluid and/or mechanically coupled to a workpiece of polymer swellable by a constituent of test fluid.

TECHNICAL FIELD

This invention relates to an analytical device, especially a sensor fordetecting and measuring quantities of materials in fluid form.

Known sensors based on a compressible polymer element containingconductive filler and depending on ‘percolation’, that is, electricalcontact between filler particles, are subject to various limitations,especially limited range of variation of electrical conductance.

PCT application PCT/GB00/02402 published as WO 00/79546 discloses asensor for chemical species or biological species or radiationcomprising:

-   -   a) a contacting head presenting a polymer composition comprising        at least one substantially non-conductive polymer and at least        one electrically conductive filler and being electrically        insulating when quiescent but conductive when subjected to        mechanical stress or electrostatic charge;    -   b) means for access of a test specimen to the head;    -   c) means to connect the head into an electrical circuit        effective to measure an electrical property of the polymer        composition.

The expression ‘polymer composition’ will be used herein to mean onecontaining polymer and conductive filler particles of metal, alloy orreduced metal oxide, and having a first level of electrical conductancewhen quiescent and being convertible to a second level of conductance bychange of stress applied by stretching or compression or electric field.More details of compositions of this type are available in PCTapplications GB98/00206 and GB99/00205, published respectively as WO98/33193 and 99/38173, the disclosures of which are incorporated hereinby reference.

We have now found advantageous sensors in which the properties of thepolymer composition can be put to practical effect. In general, thepreferred or optional features set out in PCT/GB00/002402 can be used inconjunction with the sensors according to the invention, in particular:

in the polymer composition the encapsulant polymer phase is highlynegative on the triboelectric series, does not readily store electronson its surface and is permeable to a range of gases and other mobilemolecules into the head and/or onto its surface, thus changing theelectrical property of the polymer composition.;

the contacting head may include stressing means, for example mechanicalcompressing or stretching or bending or a source of electric or magneticfield, to bring the polymer composition to the level of conductanceappropriate to the required sensitivity of the sensor;

the sensor may afford static or dynamic contacting. For staticcontacting it may be a portable unit usable by dipping the head into thespecimen in a container. For dynamic conducting, it may be supported ina flowing current of specimen or may include its own feed and/ordischarge channels and possibly pump means for feeding and orwithdrawing specimen. Such pump means is suitably peristaltic as, forexample in medical testing;

the properties of the system may change in real time, for example incontrolling an engine or chemical process or atmospheric quality;

in a preferred sensor the polymer composition may be excited by a linearor non-linear AC field. A range of techniques may be used to distinguishthe signal of interest from noise and from interfering signals, forexample—reactance, inductance, signal profile, phase profile, frequency,spatial and temporal coherence;

in another example the polymer composition is held in a transient stateby application of an electrostatic charge; then increased ionisation asa consequence of exposure to nuclear radiation changes the electricalresistivity, reactance, impedance or other electrical property of thesystem;

in a further example a complexing ionophore or other lock and key oradsorbing material is incorporated within the polymer composition. Suchmaterials include crown ethers, zeolites, solid and liquid ionexchangers, biological antibodies and their analogues or other analogousmaterials. When excited by a DC, linear AC or non-linear AC field, suchmaterials change their electrical property in accordance with theadsorption of materials or contact with sources of radiation. Suchmaterials offer the potential to narrow the bandwidth for adsorbedspecies and selectivity of the system. In a yet further example anelectride, that is a material in which the electron is the sole anion, atypical example of which might be caesium-15-crown-5 prepared byvaporising caesium metal over 15-crown-5, is incorporated within thepolymer composition. Other ionophore, zeolite and ion exchange materialsmight be similarly employed. Such a composition has a low electron workfunction, typically <<1 electron-volt, such that low DC or non-uniformAC voltages switch it from insulative to conductive phase withdecreasing time constant and increasing the bandwidth for adsorbedspecies and of the system. Such materials may be used to detect thepresence of adsorbed materials and or radiation sources.

SUMMARY OF THE INVENTION

According to the present invention there is provided a sensor forchemical species or biological species or radiation presenting to a testfluid a polymer composition comprising polymer and conductive fillerparticles of metal, alloy or reduced metal oxide and having a firstlevel of electrical conductance when quiescent and being convertible toa second level of conductance by change of stress applied by stretchingor compression or electric field, in which the polymer composition ischaracterised by at least one of the features:

-   -   (a) in the form of particles at least 90% w/w held on a 100 mesh        sieve; and/or    -   (b) comprising a permeable body extending across a channel of        fluid flow and/or affording in-and-out diffusion of test fluid        and/or    -   (c) mechanically coupled to a workpiece of polymer swellable by        a constituent of test fluid.

In aspect (a) preferably the particles are at least 90% held on a 50mesh sieve. For most purposes they pass an 18, possibly a larger e g 10,mesh sieve. They appear to be approximately spherical, of averagediameter over 150, especially over 300, microns, and usually up to 1,possibly 2, mm. They may be used with advantage in embodiments of theinvention in aspects (b) and (c). Preferred forms of the particles aredescribed below.

The particles may be random-packed in a containing vessel without orwith mutual adhesion, or supported on a yieldable framework such as foamor textile.

In aspects (a) and (b) the response of the sensor is due to the effectof the species or radiation on the polymer of the polymer composition orof a supporting framework. Preferably this effect is swelling of thepolymer widening the separation between the conductive filler particlesand thus a decrease in electrical conductance. Such widening lengthensthe path of electrons through the polymer coating on the fillerparticles and thus decreases quantum tunnelling conductance.

In aspect (c) the effect of the mechanically coupled workpiece is tocompress the polymer composition, thus decreasing the separation betweenfiller particles, shortening the electron path and increasing tunnellingconductance. The workpiece may act as a mechanical member, for example apiston or lever; instead or in additional it may be may act randomly,for example as particles mixed with particles of the polymercomposition. Evidently the operation of aspect (c) can oppose theoperation of (a) or (b); this is, however, applicable in specialisedconditions.

Each sensor includes means for ohmic connection of the polymercomposition to an electrical circuit. To match the very long curve ofconductance versus applied stress, the circuit preferably includesfield-effect-transistors and logarithmic amplification. To distinguishanalytes by rate of change of conductance, differential circuitry may beused. Ohmic connection can be conveniently provided by enclosing apermeable block of polymer composition between grids wholly or partly ofohmic conductive material, for example metal, or light metal mesh backedby plastic or ceramic, or metallised ceramic. If the polymer compositionis in sheet form stretched across the channel, spaced ohmic conductorsmay be for example mechanically held in contact with it or formed on itas a coating such as a metal-rich paint or vapour-deposited layer.Intermediate and/or external conductors, ohmic or not, may comprise apre-stressed polymer composition, possibly on a polymer or textilesupport.

Each sensor according to aspect (a) or (b) further includes means tostress the polymer composition to an initial level of electricalconductance susceptible to measurable change as a result of contact withthe test fluid. This is conveniently provided by compressing the body bydisposing the body in a tube between grids and squeezing the gridstogether, suitably by the action of an internal sleeve slidabletelescope-wise in the tube, possibly using a micrometer. For sheet formcomposition stressing is suitably by stretching by a sock-donning actionor by bending unsupported or supported e g over a former or by deforminga disc to a shallow cone or spheroid.

For each the polymer composition may be stressed before contacting. Thismay be effected for example by suitable formulation of the compositionsuch as mixing in presence of a volatile liquid removal of whichcompresses the composition to conductance. In another method itsstress/resistance response may be measured after contacting and comparedwith a standard, typically the same or a duplicate head in equilibriumwith blank fluid. Mechanical means of pre-stressing may be for examplescrew, hydraulic, piezo-electric, magnetic and thermal expansion e gusing a bimorph.

A preferred composition is in the form of particles coated with polymer.The coating may be shrunk-on, possibly with compression sufficient forpre-stress to conduction. The particles may be for example granules asdescribed herein, agglomerates thereof or comminuted bulk composition.The coating is permeable to analytes to which the sensor is to beapplied. It is also thin enough to permit electrical conduction byquantum tunnelling as described below or, possibly at greater thickness,by conductive filler such as in the composition and/or carbon. Theshrunk-on polymer is suitably a thermoset, for example epoxy, maleimideor 3-dimensional olefin resin.

The pre-stressed particles may be used in a loose-packed bed as in FIG.1( a), 3(c) or 4(d) below. Conveniently they may adhere together,possibly with mild compression, in a shaped unit as in FIG. 7 below.Thus a series of units may be set up, differing in analyte response butinterchangeable in the sensor structure.

For aspect (c) the option is available to start at non-conductance or‘start-resistance’ as an alternative to initial stressing toconductance, and use the swelling of the polymer element to produce orincrease conductance in the polymer composition.

Instead of or in addition, each sensor may be brought to the first levelof conductance by an applied voltage and/or an electrostatic orradiative or magnetic field. The first level of conductance of thepolymer composition is preferably substantially zero or at a low value(‘start-resistance’) sufficient to indicate that the sensor is incircuit.

The sensor may be used in combination with external means to modify itsresponse. For example the fluid may be contacted, upstream of the head,with a sorbent effective to remove one trace material, leaving anotherto be determined by the sensor. In a particular embodiment the sorbentmay be disposed close to the sensor head, thus avoiding a separatetreatment step. Conversely a sorptive source of co-determinable materialmay be used. Drying and (respectively) humidification are examples.

In another example, suitable for very low concentrations of tracematerial, such a sorbent may be used to take up and store the wholeamount of such material over a time period, then heated to desorb thematerial and pass it to the sensor.

Combination set-ups used in analysis may include, for example:

-   -   means to inject a known content of a known trace material, e g        for calibration or co-sorption;    -   two sensors in parallel, one calibrated as reference;    -   an array of two or more sensors in series or parallel, for        simultaneous detection of different trace materials;    -   a series succession of separately wired sensors constituting a        chromatographic column;    -   supply of blank fluid, with changeover switching, to regenerate        the sensor;    -   local heating to change specificity or assist regeneration; for        this purpose the polymer composition or swellable polymer or        sorbent may contain a heating coil or the polymer composition        may be heated by feeding electricity to it up to its PTC        temperature;    -   a substantial number of devices in parallel, with fluid        changeover switching, to afford longer time for regeneration if        required;    -   miniaturisation;    -   feedback control of stress levels;    -   computerised recording, comparing, transmitting.

Swellable polymers in aspect (c) and sorbents used to modify theresponse of the sensor may be selected from for example:

Structure-Wise:

compressed, sintered or bonded particulate;

coatings on high-surface support such as honeycomb or foam or textile;

ion-exchange resins;

chromatographic agents;

Chemical Composition:

chosen according to solubility parameter or chemical reactivity, forexample for hydrocarbons, oxygenated hydrocarbons, acidics, basics,water, viruses, bacteria.

Any of the sensors may of course be used to determine the presence of ananalyte or register the absence of an analyte that ought to be present.

In the polymer composition the metal, alloy or reduced metal oxide maybe for example in one or more of the following states:

-   -   (i) on a resilient polymer structure ‘naked’, that is, without        pre-coat but possibly carrying on its surface the residue of a        surface phase in equilibrium with its storage atmosphere or        formed during incorporation with the polymer;    -   (ii) on a resilient polymer structure carrying a thin coating of        a passivating or water-displacing material or the residue of        such coating formed during incorporation. This is similar to (i)        but may afford better controllability in manufacture;    -   (iii) on a resilient polymer structure very thinly        polymer-coated so as to be conductive when unstressed This is        exemplified by granular nickel/polymer compositions of so high        nickel content that the physical properties of the polymer are        weakly if at all discernible. As an example, for nickel starting        particles of bulk density 0.85 this corresponds to a        nickel/silicone volume ratio (tapped bulk:voidless solid)        typically well over about 10. Material of form (iii) can be        applied to the resilient structure in aqueous suspension. The        polymer may or may not be an elastomer. Form (iii) also affords        better controllability in manufacture than (i);    -   (iv) polymer-coated but conductive only when stressed. This is        exemplified by nickel/polymer compositions of nickel content        lower than for (iii), low enough for physical properties of the        polymer to be discernible, and high enough that during mixing        the nickel particles and liquid form polymer become resolved        into granules rather than forming a bulk phase. The relatively        large granules preferred may be obtained by suitable control of        mixing conditions, possibly with sieving and re-work of        undersize An alternative would be to use particles made by        comminuting material as in (v) below. Unlike (i) to (iii),        material (iv) can afford a response to deformation within each        individual granule as well as between granules, but ground        material (v) is less sensitive. Material (iv) can be handled in        aqueous suspension;    -   (v) embedded in bulk phase polymer, i e with sufficient polymer        present to form a continuous polymer structure. This can be made        by single-stage mixing or by mixing material (iv) with further        polymer of the same or different type. Like (iv), material (v)        is conductive only when stressed.

The general definition of the preferred polymer composition exemplifiedby (iv,v) is that it exhibits tunnelling conductance when stressed. Thisis particularly a property of polymer compositions in which a fillerselected from powder-form metals or alloys, electrically conductiveoxides of said elements and alloys, and mixtures thereof are inadmixture with a non-conductive elastomer, having been mixed in acontrolled manner whereby the filler is dispersed within the elastomerand remains structurally intact and the voids present in the startingfiller powder become infilled with elastomer and particles of fillerbecome set in close proximity during curing of the elastomer. Preferredconductive filler particles have a secondary structure including a spikyor dendritic surface texture, evident from a bulk density less than onethird of their solid density before incorporation into the polymercomposition. Polymer compositions exhibiting tunnelling conductance arethe Quantum Tunnelling Composites available from PERATECH LTD,Darlington, England, under the trade name ‘QTC’.

For a sensor available for more than one determination, the polymercomposition is reversibly convertible between the levels of electricalconductance. However, in specialised uses this may not be necessary:then the composition may be non- or incompletely-convertible.

The invention includes items characteristic of its aspects, such as maybe separately marketable, especially the QTC elements described withreference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in sectional elevation examples of a sensor in which thetest sample flows through the polymer composition;

FIG. 2 shows in perspective or sectional elevation or plan sensors inwhich the test sample acts on polymer composition by way of diffusion;

FIG. 3 shows in sectional elevation sensors in which the test sampleacts on a swellable polymer member, which in turn applies a stress tothe polymer composition;

FIG. 4 shows in sectional elevation or perspective sensors based onpolymer composition in a specific structural form;

FIG. 5 shows in perspective more complicated laboratory machines basedon the sensor; and

FIG. 6 shows graphically and in a Table the response of 3 sensors tovarious analytes.

FIG. 7 shows in sectional elevation a sensor in which the test sampleflows through an immobilised bed of aggregates of polymer compositiongranules pre-stressed to conductance by shrunk-on thermoset;

In these drawings, where a fluid flow direction is indicated, this isfor convenience of description, not for technical limitation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1( a), the contacting head comprises fluid flow tube10 presenting an internal surface inert to the fluid to be contacted andelectrically insulating, at least in a region to be described. At thelower end of tube 10 is tube 12 fixed in position by means not shown andformed at its upper end with rigid grid 14. Tube 12, at least at theperiphery of grid 14, fits fluid-tightly within tube 10. At the upperend of tube 10 is slidable tube 16, which is movable up or down byfine-adjustable means such as a micrometer (not shown), and is formedwith rigid grid 18 suitably made of frit or gauze. Like tube 12, tube 16fits fluid-tightly within tube 10. Grids 14 and 16 are electricallyconductive, at least on the side respectively upwards and downwards andact as electrodes connected (by means not shown) to an externalelectrical circuit. The grids may thus be made of metal, such as ametal, for example as woven wire, foam or sinter, or metallised polymeror ceramic. The grids and the surrounding region of tube 10 enclose afluid-permeable body 20 of QTC nickel/silicone polymer compositioninsulating when quiescent but conductive when compressed, to an extentdependent on the extent of compression. Body 20 may comprise for examplerandom-pack granules, possibly mutually adhering, of the composition ora structure such as foam or cloth formed of or containing suchcomposition.

To use the sensor, a steady flow of reference fluid, for example drypure air or of pure water, is set up; then tube 16 and thus also grid 18is adjusted downwards until the external circuit registers a change inresistance from a starting value to a lower value due to conduction bythe polymer composition. Then the fluid is changed to the sample to beanalysed. Resistance is measured allowing time to reach a steady state.

A modified version of this sensor is shown in FIG. 3( c) below.

Referring to FIG. 1( b), the head is in fluid-tight contact at theoutlet end of a fluid-flow tube (not shown) and presents to the tube thecentral portion of sheet 110 of QTC material, which is self-supportingas a result of initial nickel/silicone ratio or of dispersion ofnickel-rich nickel/silicone granules in a fluid-permeable supportmembrane or e g textile or foam and may be micro-perforated to easefluid flow. Sheet 110 is supported from its underside by mutuallyinsulated round-ended members 112 (fixed) and 114 (adjustablehorizontally), over which it extends externally and to which it is fixedby clamps 116 and 118, which are electrically conductive and act aselectrodes. The distance between members 112 and 114 is adjustable bymeans not shown to stretch sheet 110 to give a level of electricalconductance appropriate to the sensitivity required. Sheet 110 isconveniently rectangular, to simplify the stretching mechanism.

Referring to FIG. 1( c), in a modification of the device of 1(b) thesheet 130 of QTC material has a dished profile and is supported betweenmembers 132, which are not mutually adjustable and convenientlyrepresent a diametral section of a tube such as a hollow cylinder.Members 132 are mutually insulated by being made of or coated withinsulator or being parts of a split cylinder. Stretching of sheet 130 isby downwardly advancing fluid flow tube 134 into the dished portion ofsheet 130. Tube 134 and members 132 are co-axial.

In the sectional elevations of FIG. 2, items 266, 267, 274 and 276 are,for the sake of clarity, shown unshaded.

Referring to FIG. 2( a), a simple contacting head for lengthwise flow offluid [horizontally or perpendicular to the plane of the paper]comprises a sheet of QTC material 210 supported between metal clamp bars212 which also are electrodes providing for external electricalconnection. The head is installed in a fluid flow channel by fittingover the shoulders 214 of an insulating substrate bar 216 formed on thewall of the channel. Sheet 210 may be pre-stressed to an appropriatelevel of conductance; alternatively or additionally substrate bar 216may be split at 218 and provided with means such as a fine screw toadjust the separation of its two parts. A sensor of similarconfiguration is shown in FIG. 4( c) below.

Referring to FIG. 2( b) a fluid flow channel (not shown) carries alongat least one wall and transverse to the direction of fluid flow, aseries of ridge-shaped members 220 each presenting to the fluid a narrowsensitive region 222 of QTC material in sheet form stretched overnon-conductive former 224. Former 224 is hinged at 226 to provideadjustment of the extent of stretch. Each end of narrow region 222carries an evaporated metal connective member 228, from which an ohmicconductor can be connected to an external electrical circuit. Thestretchable polymer composition may be for example nickel in enoughsilicone rubber to give a self-supporting sheet, or nickel-richnickel-silicone granules carried by stretchable polymer sheet or foam ortextile such as LYCRA™.

FIG. 2( c) relates to an alternative form of 2(b). Here the ridge-shapedmember 240 extends from an aperture in substrate 241, to which it isclamped at its extremities. The sensitive region 242 of member 240 is atthe apex of the ridge and the necessary stretch is applied by adjustmentof edge former 244. Electrical connection to region 242 is by way ofmetal electrodes 246 applied by evaporation.

FIG. 2( d) relates to a flow pattern similar to 2(b) and 2(c) butmodified to provide the sensitive material in cones instead of ridges.Sheet-form QTC material 260 is shaped and stretched over former 264projecting through insulating disc 266 to give sensitive region 262 inthe path of flowing fluid. The conductance of region 262 is measuredbetween metal electrodes 268 formed on disc 266 by evaporation andbearing on region 262.

FIG. 2( e) is similar except that the insulating disc, now 267, isformed with a cylindrical aperture, the edges of which support needleelectrodes 269 embedded in region 262 of sheet 260.

FIG. 5 below shows how devices according to FIGS. 2( c) to 2(e) can beassembled into a multiple analyser.

FIG. 2( f,g) show modifications in which more scope for stretchadjustment is provided. FIG. 2( f) corresponds to FIG. 2( d) but differsin that substrate 263 carrying conical former 264 is replaced byperforated plate 274 and the function of former 264 is provided byheight-adjustable piston 276. FIG. 2( g) differs in the same way fromFIG. 2( e). Since piston 276 is structurally separate from plate 274, asensor using it can with relative ease be modified to fluid through-flowoperation, by making it from fluid-permeable material and providing forfluid feed to its lower end.

In FIGS. 2( h) (sectional elevation) and 2(i) (plan) the sensorcomprises a fluid flow channel 280, a wall of which presents to thefluid one side of a humped area 282, which is the convex end ofU-section folded sheet 284 of QTC polymer composition. Sheet 284projects from a recess bounded by walls 286 and bears against metalelectrode bars 288 bridging the recess with sufficient force due to itsown elasticity, possibly aided by part-closure of the recess and/or byupward applied force, to make electrical contact. The fold in sheet 284provides an electrically conductive track between bars 288 by virtue ofstretching on its outer side and compression on its inner side. Each bar288 is electrically connected by bolts 290 to a different side of therecess, with mechanical non-conducting connection to the other side viainsulating block 292.

Referring to FIG. 3( a), the sensor comprises a fluid-flow channel 310indicated generally, carrying at least one head consisting, in orderfrom bottom upwards, of:

-   -   rigid substrate 312;    -   layer 314 of QTC material coated top and bottom with        fluid-impermeable metal 316 applied by evaporation as electrodes        to be connected to external circuit by wires 318;    -   thin layer 320 of swellable polymer; and    -   rigid permeable cover 322 made of non-swellable material such as        metal or ceramic foam or frit.        Cover 322 is fixed against up-and-down movement between it and        rigid substrate 312. In use, fluid diffuses into polymer layer        320 and causes it to swell and compress QTC layer 314, thus        increasing its conductance in proportion to the extent of        swelling. The specificity of response can be changed by changing        polymer layer 320. The sensor can occupy a substantial length of        channel 310, or possibly a plurality of heads containing        different polymer layers 320 can be disposed along a fluid        channel, to provide simultaneous determination of different        trace constituents.

FIG. 3( b) shows a sensor on the same principle as 3(a) but withenhanced sensitivity. The area of action of swellable polymer layer issubdivided by struts 313. Between each pair of successive struts 313 isdisposed polymer layer 321, overlying block 315 made of conductivematerial such as metal, tapered downwards to bear on QTC layer 314.External electrical connections are to each block 315 and via substrate312 to the evaporatively metal-coated QTC layer 314 as a whole. Sincethe polymer composition used has zero or low conductance in its plane,layers 321 in this sensor can be of different polymers, for sensitivityto different trace constituents in the fluid.

Referring to FIG. 3( c), the sensor is similar to that of FIG. 1( a),but grid 18 (now numbered 22) is separated from tube 16 and is movableup and down. Grid 22 may comprise electrically conductive material andact as an electrode, but this is not necessary if QTC block 20 carries aconductive coating such as evaporatively applied metal. Above grid 22 isdisposed block 24 of permeable swellable polymer as for examplerandom-packed particles, open-cell foam, cloth or honeycomb: suchpolymer is chosen to be absorptive of, and thus swollen by, aconstituent of the fluid to be analysed. Above polymer block 24 isdisposed porous ceramic frit 26, distributing the generated stress overblock 24. This sensor is used in the same general manner as 1(a).However, particular modes of operation are available:

-   -   1. block 24 can remove from the fluid a constituent that is of        no interest, thus preventing it from masking other constituents        that are to be determined by reference to change of electrical        resistance of body 20;    -   2. block 24 can swell and apply pressure to body 20, thus        decreasing its resistance. This enables the sensor to react to a        constituent that is inert to the polymer component of body 20,        and thus broadens the scope of use of the sensor without        changing the polymer component of body 20;    -   3. if the trace material is present in very low concentration,        it may be stored in block 24 over a relatively long time, then        expelled by heating (means not shown) over a short time, thus        passing a more substantial quantity to body 20 to affect its        conductance.

Referring to FIG. 4( a), in a fluid channel indicated generally at 410is disposed block 412 of fluid-permeable polymer composition consistingof granular QTC nickel/silicone (weight ratio 7:1; volume ratio 0.824:1of solid nickel within the composition), dispersed in collapsed siliconefoam, as described in application PCT/GB/02402). Upstream and downstreamof block 412 are placed rigid metal frit electrodes 414, and these areheld in contact with block 412 by adjustable bolts 416. Block 412 may beelectrically non-conductive or weakly conductive (‘start-resistive’) asinstalled, then brought to conductance by compression by tighteningbolts 416. Alternatively block 412 may be conductive as installed, forexample by more strongly collapsing its foam structure and/or by usinginitially conductive nickel/silicone of higher nickel content or shrunkduring cross-linking: then bolts may be used to increase startingconductance further. Block 412 and electrodes 414 may be supported in anouter sleeve for insertion into flow channel 410, with O-ring sealsmating with the wall of the channel.

The sensor of FIG. 4( b) is similar to that of FIG. 4( a) but can, owingto longitudinal instead of transverse flow, afford a longer residencetime of fluid. The gas flow channel is suitably of rectangularcross-section, at least in the region of the sensor. Block 413 can be ofthe same composition as in FIG. 4( a) and is disposed betweennon-permeable metal electrodes 415 with compression adjustable by bolts417. Alternatively, to fit a cylindrical channel, compression can beadjusted by a worm-driven tubing clip.

A sensor designed to use the principle of FIG. 4( b) is shown inperspective view in FIGS. 4( e) and 4(f) below.

The sensor of FIG. 4( c) affords a relatively short residence time. Itis similar to FIG. 2( a) but provides throughflow of fluid. Thesensitive element is sheet 430 of foam-supported nickel/silicone QTCgranules as in FIG. 4( a), supported by non-conducting fixed substrate432 and horizontally movable substrate 434, adjustment of which variesstretch and thus conductance of sheet 430. At the extremities of sheet430 are electrodes 436, clamped into electrical contact with sheet 430by bolts 438.

FIG. 4( d) shows a sensor applicable to an outlet pipe 440. It comprisesouter framework 442 having fluid-permeable wall region 444, supportingcylindrical block 446 formed internally with axial passage sized to fitsnugly over the end of pipe 440 and closed at its downstream end at 448,so that fluid flow is outwardly through region 444. Pipe 440 may beformed with a perforated downward extension controlling the distributionof fluid into block. Block 446 is made of the same foam-supportedpolymer composition as in FIG. 4( a). Above block 446 and in electricalcontact with it is hollow metal cylinder 450 fitting snugly over pipe440 and fixed in relation to block 446 within framework 442. Below block446 and in electrical contact with its downstream end 448 is metalcylinder 452, which is movable up and down within framework 442 toadjust the conductance of block 446.

In FIG. 4( e,f) items 413, 415 and 417 correspond to those shown in FIG.4( b). Electrodes 415 are made of stainless steel and their position inrelation to QTC block 413 is adjustable by means of bolts 417. They areremovable or replaceable by sliding axially of cylinder 420. The wholeunit is assembled in outer cylinder 420, suitably made of ‘PERSPEX’acrylic polymer, formed with grooves housing O-rings 422 to form a sealwhen inserted into a cylindrical fluid flow channel.

Referring to FIG. 5, sketches (a,b) show how devices according to FIGS.2( c) to 2(e) can be assembled into a multiple analyser. In FIG. 5( a)rigid substrate 263 formed with cones 264 is aligned with QTC sheet 260and holes 265 of insulating disc 266,267, possibly on a shaft passingthrough holes 272. The three items are then pressed together.

FIG. 5( b) show a modifications of FIG. 5( a) in which more scope forstretch adjustment is provided. Now substrate 263 carrying conicalformer 264 is replaced by perforated plate 274 and the function offormers 264 is provided by height-adjustable pistons 276. The analyseris assembled in the same way as in FIG. 5( a).

Referring to FIG. 5( c), a miniaturised throughflow sensor 510, such asdescribed with respect to FIG. 1, 3(c) or 4(c), is mounted in each ofthe holes 512 in disc 514. Disc 514 is rotatable about bearing 516 bypowered means (not shown). The fluid inlet 518 of each sensor is fedfrom a separate source of analyte or from a rotary changeover valvesystem (not shown). Using such a valve system each sensor can operate insuccessive phases, for example, sorption, equilibration,desorption/washing.

Referring to FIG. 5( d), a system such as that of 5(c) can be operatedwith electrical instead of or additional to mechanical stress. Inposition 520 a high voltage pulse applied to the QTC material in sensor‘A’ by way of its electrodes induces conductance. Sensor ‘A’ is thenmoved to position 522 at which it is connected to a Wheatstone Bridgecircuit. Flow of analyte is started and its effect on conductance ismeasured. At the end of measurement sensor ‘A’ is moved to position 524for subsequent phases such as mentioned above, or possibly forelectrical reactivation. When sensor ‘A’ reaches position 522, a furthersensor ‘B’ arrives at position 520 and is activated by high voltagepulse and so on.

FIG. 6 reports the effect of various vapours on conductance. For thisoperation a contacting unit as described with respect FIG. 1 was used,in which block 20 consisted of QTC polymer composition as follows:

conductive filler: nickel 287 (INCO Corp) polymer ‘SILCOSET 153’ (AmberChemicals: acetoxy-cure silicone rubber with fumed silica reinforcer)nickel:polymer ratio 8:1 w/w granule size through 18 mesh, on 50 mesh.The contacting unit is connected to a source of dry nitrogen at 1 atmpressure alternatively direct or by way of a bubbler containing theanalyte in liquid form. From the upper and lower electrodes 18,14 leadsrun to a circuit comprising:

-   -   WEIR 4000 voltage source;    -   KEITHLEY 2000 multimeter (FET conductance bridge); and    -   LabVIEW software in PC.        The test was started up by feeding nitrogen, setting the input        electricity supply at 10 volts, 1 mA and adjusting tube 16 until        the conductance agreed with the intended input steadily over 15        min. Then the gas feed was switched to pass through a bubbler        containing n-hexane. As shown in FIG. 6( a,b) the resistance        increased over 10 min to 10⁴ times its starting value, much of        the increase occurring in the first 8 min, corresponding to        sorption on the silicone At 40 min the gas feed was switched        back to pure nitrogen. The resistance now decreased by a factor        of about 100 over 5 min and to its starting value in about 16        min.

The other graphs of FIG. 6 show a similar range of variation ofresistance, but differences in speed of sorption or desorption. In otherexperiments it was observed that the unit is capable of responding tothe presence of water vapour in the nitrogen.

The Table reports results for 3 sensors in which, respectively, thenickel conductive filler was dispersed in silicone, polyurethane andpolyvinylalcohol. For each determination the QTC was compressed toapproximately 20 ohms. The nitrogen flow rate was 50 ml/min, saturatedwith vapour at room temperature. In each box the resistance in ohms isgiven for 30 seconds, 60 seconds and saturation (i e no furtherincrease), the times being counted from the start of the change ofresistance. It was also observed that on stopping the supply of analytebut continuing pure nitrogen flow, the resistance decreased immediatelytowards its stating value. The sensor is therefore very effective forshowing failure of supply of a desired constituent of a fluid stream.

Referring to FIG. 7, the sensor comprises outer tube 710 formed with afluid inlet section 712 and outlet section 714. Section 714 is ofsmaller diameter than 712 and forms an annular shelf 716 at the junctionof the sections. It would be equally possible to use a tube of uniformdiameter and provide an annular insert. Shelf 716 carries a support grid718 of electrically insulating material., which in turn carriescylindrical unit 720 of mutually adhering particles each of which is anaggregate of QTC granules coated with shrunk-on thermoset epoxy resin.Unit 720 carries metal terminals 722 for external electrical connectionvia grommets not shown. Terminals 722 may be separated axially ordiametrally. Thus they may consist of metal grids top and bottom, inwhich event axial pressure is applied to ensure electrical contact. Fordiametral separation metal electrodes may be for example:

-   -   in contact with the periphery of the unit; or    -   drilled into the unit near the periphery; or    -   pressed downward on its upper surface near its periphery; or    -   pinching the unit near its periphery.

1. A sensor comprising i) a conduit for through-flow of a test fluid;ii) a porous body of granules of a polymer composition having particlesof conductive filler metal, alloy or reduced metal oxide dispersedtherein, said body having a first level of electrical conductance whenquiescent and being convertible to a second level of conductance bychange of stress applied to the body by stretching or compression orelectric field, said body being permeable to the test-fluid and disposedacross the conduit whereby, in use, said test fluid flows through saidbody; and iii) electrodes connected to said body for connection to anelectrical circuit responsive to a change in conductance of said body.2. A sensor according to claim 1 wherein a bed of granules is supportedin the conduit between a pair of perforate grids forming the electrodes.3. A sensor according to claim 2 wherein at least one of the grids ismoveable towards the other whereby the bed can be compressed.
 4. Asensor according to claim 3 wherein the porous body is held betweenclamps forming the electrodes disposed on either side of the conduit. 5.A sensor according to claim 3 wherein the bed of granules comprises afoam or textile having granules of the polymer composition dispersedtherein.
 6. A sensor according to claim 1 wherein the porous bodycomprises a sheet and wherein means are provided to stretch the sheet.7. A sensor according to claim 6 wherein the means to stretch the sheetcomprises a hollow member bearing against one surface of the sheet andmoveable in a direction perpendicular to the plane of the sheet.